A valuable control feature for automotive air conditioning systems is the detection of low refrigerant, more particularly, of a complete loss of refrigerant. The refrigerant contains lubricant which is relied upon for compressor lubrication, and thus, the absence of such lubrication can lead to catastrophic failure.
A current system which employs cycling clutch control to turn the compressor on and off uses a pressure sensing switch which disengages the clutch when system pressures drop below a certain level. This switch protects against evaporator core freezing and also protects the compressor if most or all of the refrigerant in the system is lost. Such a pressure switch is an invasive device, that is, it makes direct contact with the refrigerant and thus furnishes a potential leak path. It is desirable to reduce the number of such potential leak paths and thus improve system integrity.
A noninvasive control system has been proposed which protects against evaporator core freezing by utilizing a thermistor in the evaporator core on the air side. That system can be designed to detect partial loss of refrigerant charge but will not protect the compressor if all the refrigerant is lost, since the low charge detection requires some refrigerant in the system in order to function properly.